Field of the Invention
The present invention relates to an image sensor and an image capturing apparatus.
Description of the Related Art
In recent years, solid-state image sensors used in image capturing apparatuses, such as digital still cameras and digital video cameras, are becoming increasingly multifunctional.
Japanese Patent Laid-Open No. 2001-124984 discloses a technique to enable focus detection of a pupil-division type in a solid-state image sensor. According to Japanese Patent Laid-Open No. 2001-124984, one pixel has two photodiodes in the image sensor, and each photodiode is configured to receive light that has passed through different pupil areas of an imaging lens using one microlens. With this configuration, focus detection can be performed on the imaging lens by comparing output signals from the two photodiodes with each other. Furthermore, a signal of a captured image can be obtained by merging the output signals from the two photodiodes.
Meanwhile, Japanese Patent Laid-Open No. 2013-106194 discloses the following technique. In an image sensor in which one pixel has two photodiodes, after a reset level signal and a signal of a first photodiode are read out, a signal of a second photodiode is combined and the combined signal is read out without resetting. The signal of the second photodiode is obtained by subtracting the signal of the first photodiode from the combined signal. According to Japanese Patent Laid-Open No. 2013-106194, as the reset level signal can be used in common to the signals of the first and second photodiodes, the number of readouts of the reset level signal can be reduced by one.
In order to perform focus detection in an image sensor in which one pixel has two photodiodes as in Japanese Patent Laid-Open No. 2001-124984, it is necessary to obtain a signal from each of the two photodiodes. This gives rise to the problem that, compared with a conventional image sensor in which one pixel has one photodiode, the amount of read signals doubles, and a readout time period significantly increases. To address this problem, application of a driving method described in Japanese Patent Laid-Open No. 2013-106194 reduces the number of readouts of a reset level signal by one, thereby reducing a readout time period. However, the effect of such reduction is not considered enough, and there is demand for further reduction in a readout time period.